1. Field of the Invention
The present invention relates to an oil pump made of aluminum alloys and, more particularly, an oil pump comprising at least one rotor of a powder metallurgical aluminum alloy in a stationary casing of an aluminum alloy, which is improved in wear resistance and mechanical strength at elevated temperature.
2. Description of the Prior Art
Recently, component parts of automobiles have been made lighter as one of the countermeasures against fuel consumption of automobiles and oil pumps for automobiles are no exception to the rule. The oil pumps are usually made of iron and comprises a stationary casing produced by molding or die-casting. Thus, the oil pumps, for example, those for automatic transmission system, have a weight of 5 kg and above. If aluminum alloys are used as a material for the component parts of oil pumps, the weight complete of oil pumps would be lightened to less than 2 Kg and the weight would be reduced by about 60%. In addition, further improvement in performance of the oil pumps is expected by lightening of the component parts.
However, old aluminum alloys which have been put into practical applications cannot be applied to rotors of oil pumps because of their poor wear resistance. For example, ingot metallurgical aluminum alloys such as AC8B and A390 (hereinafter referred to as I/M Al alloys), usually used for pistons, bearings or like parts, are materials developed in consideration of wear resistance. If such I/M Al alloys are used as a material for the rotors of oil pumps, considerable wears and damages caused by pitching wear take place at tooth flanks of the rotors because of their poor resistance to sliding wear and pressing fatigue. Further, seizing wears take place at edges and peripheries of the rotor considerably because of the sliding contact between rotor and casing. In addition, at the high rotational speed of the rotors, fatigue failure takes place at the shaft joint because of lack of strength.
Further, it is impossible with the cold forging operation to produce precision parts with a complex configuration, so that the cold forging of aluminum alloys require machining. As the content of Si in aluminum alloys increases, the machinability of the aluminum alloy decreases because of increasing particle size of primary crystals of Si, resulting in lowering of strength and toughness. In addition, aluminum alloys are required to have a content of 3 to 10 percent by weight of Fe to improve the strength at elevated temperatures, but the Fe content of more than 5% causes formation of large acicular crystal structure, resulting in decrease of the toughness. Accordingly, it is impossible to produce aluminum alloys with a sufficient strength at elevated temperatures.
Powder metallurgical aluminum alloys, i.e., aluminum alloys produced by powder metallurgy with a rapidly solidified aluminum alloy powder (hereinafter referred to as P/M aluminum alloys), such as high-Si aluminum alloys containing 20 to 40% by weight of Si, are poor in the strength at elevated temperature, thus making it difficult to apply them to rotors of oil pumps for automatic transmissions, which are operated at elevated temperatures of about 150.degree. C.
In order to improve the wear resistance of the P/M aluminum alloys, attempts have been made to replace Si with hard particles such as SiC, TiC, Al.sub.2 O.sub.3 and so on. However, these P/M aluminum alloys have a problem similar to that of the high Si aluminum alloys. For example, in case of that the P/M aluminum alloy are applied to rotors of the oil pumps, the matrix thereof begins to soften when heated to more than 100.degree. C. by frictional heat. For this reason, the aluminum alloy is apt to be damaged because of lowering of strength. At the same time, the hard particles are left out by shearing force acting on the rotor during sliding movement of the rotors, so that wear resistance becomes lowered.
Also, P/M aluminum alloys of a Al-high Zn system have high strength at elevated temperatures because of their considerable age hardening, but they are poor in wear resistance. Thus, they cannot be adapted for rotors of oil pumps.
In JP-A-60-147785 and JP-A-62-124284, it has been proposed to provide protective coatings by anodic oxidation, nickel plating or chrome plating on rotors of sintered aluminum alloys to improve the wear resistance thereof. Under high-speed sliding motions of the rotors, however, the coatings come off or are damaged together with the matrix because of softening of the aluminum alloys at elevated temperature, resulting in seizing of rotor and casing. In addition, such aluminum alloys are required to control the thickness of coating accurately to improve the pump efficiency, resulting in increase in the production cost.
On the other hand, the stationary casings are required to have a coefficient of thermal expansion close to that of the rotors to provide a high pump efficiency. If the coefficient of thermal expansion of casing differs greatly from that of the rotor, a clearance between them increases with temperature, resulting in lowering of the pump efficiency. Also, the stationary casings are required to have a wear resistance close to that of the rotors to control the increase of clearance between them due to wear of the casing.
To meet such requirements, the inventors have tried to produce a casing of oil pump with a powder metallurgical material such as rapidly solidified Al-Si alloys. However, if the casing of a rapidly solidified Al-Si alloy is used in combination with the rotor of the rapidly solidified P/M aluminum alloy, it is apt to cause seizing and adhesive wear of the oil pump.